They do, in an extremely expensive system called ETCS, which everybody is aiming for but it takes a huge pile of money and years of work to ever get there. Which is why a huge portion of rail networks don't (yet) use it.
The issue is not the idea, it's that railway signalling systems have to be an interoperable, multi-vendor, tested and accredited pile of little ideas like this, that have infinite numbers of ways to go wrong and have corner cases by the metric ton. And it's not like coding a website, where you can just ignore standards and focus on your case. "Your" rails need to support other people's trains. "Your" trains need to run on other people's rails. Even in a fully closed system with your trains on your rails... who's going to be your equipment maintainer and supplier for the next 30-50 years of their operational life?
Read this article and notice how often cost and project overruns are mentioned:
> The use of ETCS is driven by deliverables, not the technology but concerns exist as to why it is costing so much. In Europe, the success of ETCS is driven by country mandate and a system authority which helps to create a stable ongoing work plan.
> There is also the question of strategic safety versus functional safety, namely fundamental versus site specific. Over-specified ETCS undermines the strategic safety and functional over scope and so drives unaffordability since the accreditation inflates the cost.
> Cost must be driven out of ETCS if it is to be the preferred system for ongoing renewals. Despite interoperability requirements, there is risk in always using the same supplier. There is no common way of delivering ETCS. Having it as an overlay system will always be expensive and not beneficial to the business case. Lessons have been learned from the application of ETCS to the Cambrian route, the Thameslink central core, and the Northern City line from Finsbury Park to Moorgate but all of these have only one type of rolling stock in day-to-day operation. Equipping a mixed traffic railway as in the ECML makes it much more difficult.
TCAS has solved this for airplanes. It wouldn't take much to modify the system for trains so that, in the least, it notifies when an unexpected train is nearby. A little more logic could allow it to verify which track each train is on.
1. Planes can determine their altitude. Trains don't know which track they're on. You'd have to re-engineer existing signalling systems designed around the inverse. Track transponders and axle counters know which trainset passed a specific piece of track, but the trains don't know which track transponders link together on the same track. They don't even know which track they'll be going down next because that's up to the signallers controlling the points, not them.
2. Planes fly in the air and can send omnidirectional radio signals with little to no interference. Trains run on the ground, often in valleys and tunnels. There is no guarantee that trains broadcasting "I am here" signals can be picked up by the next train within the stopping distance.
Many trains did broadcast "I am here" signals, but once speeds got faster, the broadcast range (about 1km) wasn't far enough for a train to stop in time before a collision, which is why trains need to communicate, using on-track wires that run the length of the track, to each other and central control. Which therefore means that the central control system needs to work, and when it doesn't, the trains have to stop.
Planes use radio signals because they don't have signalling cable following their entire route. Trains do, and it's usually much more reliable than radio.
Except, trains already have them since about 150 years ago. The problem was that the "train TCAS" requires minimum distances for detection, so to speak, below which the system can't detect anything other than itself. When the newer system went down, there were more than one train within that minimum distance, so the older backup system didn't detect anything other than itself.
They can upgrade the system so that the minimum distances are smaller. Sure. Except, such hypothetical upgraded system was already in place, since 90s, and __that was what went down__.
If you were to then ask me, okay, why did they get the trains apart before committing with the downgrade into the older system. And that would be the right question. It's such a braindead move to not do that before executing the full downgrade. It's not like trains were moving when downgrade took place and it couldn't be done, the newer system going down triggered emergency stop of all trains. Train systems are intentionally built that way. So they could (load-bearing-)just ordered the trains to get spread out so there will be enough distances between trains so the older version of "train TCAS" would work correctly. The decision to NOT do that, with hindsight, is downright reckless.
By the way, if all reports on this matter is to be believed, the required distance of the older version of the "train TCAS" in operation was in excess of 25km, or 15 miles. That's long. It can be shortened. Sure. But then again, there was such a newer system already in place since 90s that has much less minimum distances than that, which went down.
Obviously it wouldn't be "just" GPS. In the simplest example - if I choose to add a friend to "Find my Friends" on iPhone, both our locations are visible to each other via GPS when our phones are on.
... via mobile data or WiFi, you mean. Whether it uses GPS, Galileo, track measurement points, or even "driver calls the control centre to update their position" doesn't have much to do with it. What I assume you mean is redundancy so that two or more systems can communicate train positions independently, where the more relevant problem is how to get the data across in a timely manner. The driver of the stationary train called in their position and it wasn't used: it's a communication problem, not a "where am I" problem. Idk why the focus on GPS and using GPS for communication (which GPS can't do, and newer systems only do at very low speeds for emergency communication) but finding one's position is usually not the problem
I was wondering, but "earthquakes" perhaps. It's much easier to fix a canal, even with some of the water flowing.
There's a cost to pipes, but at least part of that would be offset by the structural support for the solar panels and the fact that you need to pipe less waters. Also I assume that the canals are lined in some way. I don't know, maybe the canals are just prettier to look at?
A similar problem is plaguing the Aral sea. The Amu Darya river is being drained by ineffective and leaky canals.
Not living in the US but in the Netherlands. It's quite popular to do this, because the energy is really expensive, be it electricity, be it natural gas, be it petrol. If you are able to own a single family home, you are likely to have the means to invest 4k-10k euros solar installation costs (depending on how many panels) and benefit from reduced energy bill.
In short, cheap energy might be the culprit but I don't know for sure.
I'm not sure, as this paper is very confusing in its style, but I think they try to modulate the load resistance connected to an antenna on the TX side. Since the open has a large real resistance (25k in their case), it's poorly impedance matched to the antenna and most of the noise powwr cannot be transferred to the antenna at 1.4 GHz they are using. 50 ohm load is well matched, hence the max noise power transfer to the antenna. This is all at TX.
I don't see a thermodynamic problem here. Noise modulation with switching is well known in electronics. Switched resistors and their noise is already analyzed. It is often in the context of noise parr of SNR though. They are using this noise modulation to transmit signals, which isn't too novel. I guess the transmit power is very low but this would be horrible for Shannon spectral efficiency because of the need for noise power detection, which cannot be instantenous.
I suspect GP's question is answered by the fact the system is not in thermal equilibrium. The view the antenna has is approx half of the sky whose temperature is far lower than room temp. Even indoors, the roof of your house is sufficiently transparent in RF that you are still seeing the sky behind.
If the same experiment was done in metal box with everything of constant temperature, I don't think it would be possible to transmit any bits.
Nuclear power is expensive enough on land. At sea it’ll never compete with any other energy source. They’ll be pulling hydrocarbons from the air before they do nuclear ships in earnest. Yes I have heard of nuclear carriers and subs, the only two useful applications of nuclear power at sea.
Russia has been using nuclear icebreakers for civilian purposes since the 50s, and has also used nuclear propulsion for other vessel types, as has the US.
Thanks, I am pretty knowledgeable about nuclear ships and they’re only used in specific situations because they don’t care about cost for those situations. It’s too expensive for cargo and there is no training pipeline for civilian naval reactors so this idea is dead on arrival.
I’m pro nuclear tech but the economics and systems will not support nuclear for ships right now. We are barely making headway into nuclear on land these days, let’s focus on that for now.
I really hate the fossil fuel dependency of the current aircrafts. Time will tell if there will be a place for battery electric aircraft, and if there is, will the introduction be in the west where Boeing and Airbus duopoly is killing all the competition (i.e. Bombardier CS series saga). I won't say China will be more open as it's popular to do these days on HN. One hopes some other nations with less political power but enough industrial capabilities can do this for domestic flights (like Brazil, India, Indonesia..) in the future, if the global politics continues to go in its current trend.
